Multiple input multiple output (MIMO) is technology for raising communication capacity or a signal to interference-plus-noise ratio (SINR) in proportion to the number of antennas by using multiple antennas both at a transmitting end and a receiving end. For reference, a scheme of using multiple antennas only at the transmitting end is called multiple input single output (MISO), a scheme of using multiple antennas only at the receiving end is called single input multiple output (SIMO), and a scheme of using a single antenna at both the transmitting end and the receiving end is called single input single output (SISO). MIMO technology, nonetheless, can be used to collectively refer to MIMO, SIMO, MISO, and SISO schemes. The MIMO technology is called multi-antenna technology because multiple transmit antennas and/or multiple receive antennas are used unlike a conventional antenna scheme using a single transmit antenna and a single receive antenna,
According to MIMO technology, the receiving end completes whole data by collecting fragments of data received from some antennas without depending upon a single antenna path in order to receive one whole message. As a result, MIMO technology may raise data transmission rate within a specific range or expand system range at a specific data transmission rate.
In a current radio communication environment, machine-to-machine (M2M) communication, various devices such as smartphones or tablet personal computers (PCs) requiring high data transmission rate, and various techniques have appeared and come into widespread use. This has rapidly increased the quantity of data which needs to be processed in a cellular network. Many communication operators or developers have shown an interest in MIMO technology among a variety of techniques capable of increasing data throughput and have made an effort to optimize MIMO technology because MIMO has the advantage of remarkably increasing transmission/reception performance and communication capacity even without allocating additional frequency or power.
MIMO technology may be used for transmit diversity, beamforming, spatial multiplexing, etc. Transmit diversity is a technique for raising transmission reliability by transmitting the same data through a plurality of transmit antennas. Beamforming is used to increase an SINR of a signal by adding a weight in a plurality of antennas according to a channel state. The weight may be expressed as a weight vector or a weight matrix, which is referred to as a precoding vector or a precoding matrix. Spatial multiplexing is technology capable of transmitting data at high rate without increasing bandwidth of a system by simultaneously transmitting different data through a plurality of transmit antennas. Spatial multiplexing is categorized into spatial multiplexing for a single user and spatial multiplexing for multiple users. Spatial multiplexing for a single user using MIMO is referred to as single user MIMO (SU-MIMO) and spatial multiplexing for multiple users using MIMO is referred to as spatial division multiple access (SDMA) or multiple user MIMO (MU-MIMO).
Meanwhile, the MIMO technology is divided into two transmission schemes: open-loop MIMO and closed-loop MIMO depending upon whether or not channel information about a radio channel formed between transmit antenna(s) and receive antenna(s) is shared between the transmitting end and the receiving end.